Populations of several species of gregarine parasites within a single host species, the damselfly Ischnura verticalis, were examined over the course of 1 season at 4 geographic localities separated by a maximum distance of 9.7 km. Gregarines, having a life cycle with both exogenous and endogenous stages, are subject to a wide variety of selective pressures that may drive adaptation. Gregarine species showed some specificity for host life cycle stage, i.e., Steganorhynchus dunwoodyi and Hoplorhynchus acanthatholius were most prevalent in larval hosts while Steganorhynchus dunwoodyi, Actinocephalus carrilynnae, and Nubenocephalus nebraskensis were most prevalent in adult hosts. Species prevalence and abundance differed by geographic locality. Gregarine prevalence was significantly higher in adult female damselflies than in males at 2 localities; sex differences in prevalence were insignificant for larval damselflies at all 4 localities. In larval hosts, gregarine abundance was independent of age (size). The present study, therefore, shows that pond characteristics, host life cycle stage, and adult host sex are the main factors that influence the prevalence and abundance of gregarine populations.
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